1. Field of the Invention
The present invention relates to an antenna device for, e.g., a vehicle, and more particularly to an antenna device having a balanced configuration, such as an antenna for a global positioning system (GPS antenna) of, e.g., an on-vehicle navigation apparatus, applicable to a vehicle.
2. Description of the Related Art
Generally, an on-vehicle antenna of the an unbalanced antenna type is commonly used.
For example, microstrip antennas are disclosed in Japanese Patent Application Laid-Open Nos. 5-22030, 5-192912, and Japanese Utility Model Gazette (Kokai) No. 5-6380 as unbalanced antennas.
In an unbalanced antenna, each side of an antenna plate, provided in parallel to an earth plate, must have a length equal or greater than .lambda./2, where .lambda. is a wavelength of a radio wave to be received; therefore, the size of the antenna is large.
In Japanese Patent Application Laid-Open No. 5-192912, dielectric is inserted between an antenna plate and an earth plate in order to reduce the size of an unbalanced antenna. However, raw material cost and processing cost for forming the dielectric is very high, which makes the antenna expensive.
As an antenna which can be down-sized and supplied at a low price, balanced antennas are attracting attention.
In the balanced antenna, especially in a loop antenna, the length of one cycle of the loop must be designed to match .lambda., where .lambda. is the wavelength of a radio wave to be received; accordingly, it is an advantage of the balanced antenna that the size of the antenna is reduced without using expensive dielectric. Further, the balanced antenna does not have to have an earth (earth plate), which is different from an unbalanced antenna; accordingly, it is an advantage of the balanced antenna that it is unnecessary to ground the antenna when installing the antenna on, e.g., a vehicle.
FIGS. 1A and 1B show a configuration of a conventional loop antenna. FIG. 1A is a top view of an antenna element, and FIG. 1B is a side view of the antenna element.
Generally, a balanced antenna 10 having a shape as shown in FIG. 1A has a directivity as shown in FIG. 2I in free space (i.e., substantially no metal conductor exists in periphery area).
Note, measurement of the directivity, whose result is shown in FIG. 2I, is performed in the polar coordinate system as shown in FIG. 1D. Further, in FIG. 2I, E.sub.74 (shown by a black diamonds) and E.sub..phi. (shown by squares) show sensitivity of the antenna 10 measured when an originator of a radio wave is at angles .theta. and .phi., respectively, as in FIGS. 1E and 1D. More specifically, E.sub.74 is the sensitivity measured when the loop antenna 10, which is the subject of the measurement, is arranged at the origin in the horizontal plane, and a dipole antenna 200, which is the originator of a radio wave, is arranged perpendicular to effective pieces 10e of the antenna 10, as shown in FIG. 1E. For example, the dipole antenna at .theta.=90 degrees is orthogonal to the effective pieces 10e of the antenna 10 and normal to the horizontal plane. Further, when .theta.=0 degree, the dipole antenna is orthogonal to the effective pieces of the antenna 10, but parallel to the horizontal plane. E.sub..theta. (.phi.=90 degrees) is the sensitivity measured when the loop antenna 10, which is the subject of measurement, is arranged on the origin in the horizontal plane, and the dipole antenna, which is the originator of a radio wave, is arranged parallel to the effective pieces 10e of the antenna 10 as shown in FIG. 1F. Thus, the dipole antenna is parallel to the effective pieces 10e when .theta. is, e.g., 0, 45, or 90 degrees.
The inventors of the present invention carried out an experiment on the loop antenna 10, shown in FIGS. 1A and 1B, under conditions as shown in FIG. 1C. Specifically, a metal conductor 100 is placed at a distance D from the antenna 10 in parallel to the antenna 10. The metal conductor 100 practically corresponds to metal of harness and body of a vehicle.
As a result of the experiment as shown in FIG. 1C, performance of the antenna 10 was affected by the peripheral conductor, such as the harness and body of the vehicle (i.e., the conductor 100), and the directivity of the antenna 10 was greatly deformed from the directivity shown in FIG. 2I.
FIGS. 2A to 2H show the directivity of the antenna 10 when the antenna 10, a loop balanced antenna shown in FIG. 1A, is placed at a distance D from the metal conductor 100, which acts as an earth plate of infinite size, where the distance D is changed. Note, the sensitivity in FIGS. 2A to 2H is shown in the same manner as FIG. 2I.
Referring to FIGS. 2A to 2H, the balanced antenna 10 shows relatively preferable characteristics as shown in FIGS. 2D and 2E; however, the directivity of the antenna 10 is often affected by the metal conductor 100, as shown in FIGS. 2F to 2H, thus it is not substantially suitable for practical use.
Further, the directivity of the antenna 10 may be affected by relative positions between the antenna 10 and the conductor 100, and would change greatly in response to slight displacement of the conductor 100 or slight misplacement of the mounting position of the antenna 10. Further, in an antenna, such as a GPS antenna, which requires directivity in a specific direction (upward, in the GPS antenna), when the distance to a conductor is changed due to the misplacement of the mounting position of the antenna, the required directivity is not achieved because of the effect of the conductor.
Especially, for mounting the GPS antenna on a vehicle, it is necessary to arrange a balanced antenna element of the GPS antenna within an instrument panel which includes many metal conductors, such as an instrument panel member and a harness; therefore, the foregoing problem is very serious.
The applicants of the present invention suggested a balanced antenna having a reflector in Japanese Patent Application Laid-Open No. 3-72702. However, the reflector is aimed at preventing a radio wave, originated from the balanced antenna, from reaching driver and passenger, and not at solving the foregoing problem.